The present invention generally relates to xerographic printers and more particularly to a xerographic serial printer for recording an image on a recording sheet based upon a toner image formed on a recording drum in correspondence to an electrostatic latent image.
In recent years, there is a demand for low cost and compact xerographic printers. In order to meet such a demand, xerographic serial printers are developed. In a xerographic serial printer, a xerographic processing head is held on a carriage for recording images. Xerographic printers generally provide a higher quality image recording, while there is a persisting demand for higher recording quality and higher recording density.
FIGS. 1A and 1B show the essential part of a conventional xerographic serial printer including the carriage respectively in a plan view and in an oblique view.
Referring to FIGS. 1A and 1B, the printer includes a carriage 11 that in turn includes a carrier member 13 and a xerographic processing head 12 mounted upon the carrier member 13. Further, the carriage 11 has a guide part 14 in engagement with a guide shaft 15. Thus, the carriage 11 is moved by a motor not illustrated over a platen 16 in a direction perpendicular to a sheet feed direction.
At both sides of the carriage 11, there are provided feed rollers 17a and 17b for feeding a sheet 18, wherein the feed roller 17b in the downstream side of a sheet feed path incorporates therein a heat source for fixing the toner image transferred upon the recording sheet 18. It should be noted that the feed rollers 17a and 17b are provided such that the sheet 18 is transported horizontally between the carriage 11 and the platen 16.
The xerographic processing head 12 includes, as indicated in FIG. 1B, a recording drum 21 rolling in the moving direction of the carriage 11, a precharger 22 for charging the surface of the recording drum 21, an exposure unit 23 for forming an electrostatic latent image on the surface of the recording drum 21, a developing unit 25 for developing the electrostatic latent image on the recording drum 21 by means of toners 24 to form a toner image, and a cleaner 26 for removing the toners remaining on the recording drum 21 after the toner image is transferred to the recording sheet 18.
In operation, the xerographic printer records an image on the recording sheet 18 by the xerographic processing head 12 while moving the carriage 11 in the direction perpendicular to the sheet feeding direction. Further, the recording sheet 18 is transported in the sheet feeding direction by means of the feed rollers 17a and 17b. It should be noted that the xerographic processing head 12 is provided above the recording sheet 18, and the recording head 12 transfers the toner image on the recording drum 21, formed as a result of the development of the electrostatic latent image thereon by way of the development unit 25, by applying a predetermined voltage to the platen 16. As already noted, the electrostatic latent image is formed on the recording drum 21 by means of the precharge unit 22 and the exposure unit 23.
After one line of recording is completed on the recording sheet 18 by scanning the carriage 11, the feed rollers 17a and 17b feed the recording sheet by a predetermined amount. Further, the carriage 11 returns to a predetermined home position corresponding to the origin. The toner image thus transferred to the recording sheet 18 is then fixed thereon as a result of heating by the feed roller 17b.
By repeating the foregoing recording process for one line a plurality of times, a desired image is recorded on the recording sheet 18.
In the conventional printer of FIGS. 1A and 4B, it should be noted the carriage 11 is located above the recording sheet 18 and achieves the recording on an upper major surface of the recording sheet 18 while being moved laterally, with respect to the sheet feeding direction, at a high speed. Thus, there is a tendency that the toners 24 held in the head 12 spreads on the recording sheet to form spots. Further, there is a tendency that the density of recording becomes non-uniform along the path of the carriage 11. In addition there occurs a problem, when the carriage 11 stops on the recording sheet 18, in that the toners 24 fall on the recording sheet 18 to form spots.